Various communication systems experience bandwidth limitations. For example, recently, the Federal Communication Commission (FCC) allocated twelve and one-half (12.5) MHz of the electromagnetic spectrum for satellite digital broadcasting to XM Satellite Radio, Inc. of Washington, D.C. (XM) and 12.5 MHz of the electromagnetic spectrum to Sirius Satellite Radio, Inc. of New York City, N.Y. (Sirius). Both satellite digital audio radio (SDAR) system companies provide subscription-based digital audio that is transmitted from communication satellites, and the services provided by these and other SDAR companies are capable of being transmitted to both mobile and fixed receivers on the ground.
In order to improve satellite coverage reliability and performance, SDAR systems currently use three (3) techniques that represent different kinds of redundancy (known as diversity). The techniques include spatial diversity, time diversity and frequency diversity. Spatial diversity refers to the use of two (2) satellites transmitting near-identical data from two (2) widely-spaced locations. Time diversity is implemented by introducing a time delay between otherwise identical data and frequency diversity includes the transmission of data in different frequency bands. SDAR systems may utilize one, two or all of the techniques. The limited allocation of twenty-five (25) megahertz (MHz) of the electromagnetic spectrum for satellite digital broadcasting has created a need in the art for a technique that increases the amount of data that may be transmitted from communication satellites to receivers in SDAR systems.
Hierarchical modulation (HM) has been designated as an alternative to the conventional modulation methods (e.g., QPSK, 16-QAM and 64-QAM) for digital video broadcasting-terrestrial (DVB-T). In a system that utilizes HM, autonomous DVB-T multiplexes are transmitted on a single TV frequency channel. HM has also been implemented in other communication systems. Unfortunately, the implementation of HM within a communication system has generally required complex receivers to demodulate the extra secondary (Level II) data. One solution for demodulating secondary data has been to demodulate/decode the primary data stream, re-encode/re-modulate the primary data stream and subtract the primary data stream from the received data stream to provide the secondary data.
Another method to demodulate secondary data from a received data stream has required taking a distance measurement of a received constellation point from four points on the real/imaginary (I/Q) plane. It should be appreciated that the distance measurement requires multiplication, subtraction and implementation of a square root function. Yet another technique has required demodulation/decoding of a primary data stream. The primary data is then re-encoded and is then utilized as a new offset detection boundary. Unfortunately, all the described techniques for demodulating secondary data have required relatively complex receivers.
What is needed is a technique for demodulating secondary data associated with a hierarchically modulated data stream that reduces the complexity of a receiver.